The general purpose of a rain gutter cover is to prevent leaves and debris from entering rain gutter and downspout systems, while allowing free flow of rain water through the same.
There are many types of screens, hoods & inserts which attempt to accomplish the above, but they all have their faults.
The two most popular coverings are screens (wire or expanded metal) and full hoods with an exposed, unprotected opening across the full vertical front of the hood.
Screens allow the water to pass through from the very beginning, but by the time the water passes over approximately the first 1″ of screening, it has all entered the gutter system, causing the front of the screening to eventually clog with leaves and debris because there is no water left to wash it clean. Eventually, the clogging backs up, covering even the first 1″ of the screening causing water to overflow the front, and sometimes the back, of the gutter. In time this build-up on the screening mulches and sifts through the screening and even plugs the gutter system, compounding the problem.
Hoods, on the other hand, keep most leaves and debris from entering the gutter system because they are constructed of solid material. The front edge of a hood is rounded, with an unprotected horizontal opening across the bottom. The water travels downward across the hood and clings to the rounded front, flowing into the front opening, while washing much of the leaves and debris over the front. However, soft leaves and debris can, and do, also follow the front contour of the hood with the water, thus entering the gutter system. Also, hoods that slip between layers of roofing can not be installed on homes with rigid roofing that extends down to the top of the gutter. This would cause a backward tilt to the hood, as the front of the hood would stand up higher than the back because of the rolled vertical front. Other hoods stand taller, requiring the back to lay on the roofing one to two tabs up the roof. When this type is installed on rough wood shakes it creates an unsightly and uneven back/top edge, allowing leaves and debris to snag on this edge as they attempt to wash down onto the hood.